Turbochargers are generally well known in the art. Basically, a turbocharger is a centrifugal air pump that is driven by the engine's exhaust gas. The turbocharger forces an additional amount of air or air-fuel mixture into the engine, which aids in the increase of combustion pressure and engine power. This allows a relatively smaller engine to deliver acceptable fuel economy during normal driving conditions, and to have the increased power characteristics of a larger engine when needed. Turbochargers have been incorporated into both gasoline and diesel engines, for both commercial and passenger vehicle applications.
Examples of turbocharger technology can be found with reference to commonly assigned U.S. Pat. No. 6,089,019 to Roby; U.S. Pat. No. 6,263,672 to Roby et al.; U.S. Pat. No. 6,415,846 to O'Hara; U.S. Pat. No. 6,543,228 to Deacon; U.S. Pat. No. 6,694,736 to Pfluger; U.S. Pat. No. 6,709,160 to Ward et al.; U.S. Pat. No. 6,715,288 to Engels et al.; U.S. Pat. No. 6,802,184 to Huter et al.; and U.S. Patent Application Publication Nos. 2003/0206798 to Allmang et al.; 2004/0037716 to Jaisle; 2004/0062645 to Decker et al.; 2004/0088976 to Jaisle; 2004/0134193 to Klingel; 2004/019447 to Roby; and 2004/0197212 to Roby, the entire specifications of all of which are expressly incorporated herein by reference. Other examples of turbocharger technology can be found with reference to U.S. Pat. No. 4,294,073 to Neff; U.S. Pat. No. 6,079,211 to Woollenweber et al.; U.S. Pat. No. 6,311,493 to Kurihara et al.; and U.S. Patent Application Publication No. 2005/0086936 to Bucknell et al., the entire specifications of all of which are expressly incorporated herein by reference.
A recent development in turbocharger technology has been regulated two-stage or sequential turbochargers, in which two relatively smaller turbocharger units are used instead of a single relatively large conventional turbocharger. These regulated two-stage turbocharger systems typically employ a relatively smaller, high-pressure turbocharger unit in series with a relatively larger, low-pressure turbocharger unit. Unfortunately, conventional regulated two-stage turbochargers still exhibit unsatisfactory characteristics, especially during the overlapping transition period when the turbo speed of the smaller high-pressure turbocharger unit rapidly decreases and the turbo speed of the larger low-pressure turbocharger unit rapidly increases. This phenomenon is believed to be a momentary loss of torque that occurs before the low-pressure turbocharger unit has reached its particular requisite turbo speed. This “drop” in torque during the transition period is quite noticeable and is objectionable to many drivers, especially those driving expensive, high-performance luxury models.
One solution to this problem can be found in commonly-assigned U.S. Provisional Patent Application Ser. No. 60/635,769 to Grissom et al., the entire specification of which is expressly incorporated herein by reference. Grissom et al. described a regulated two-stage turbocharger system that includes high-pressure and low-pressure turbocharger units in communication with one another. The turbocharger system includes a valve system having valve members that are independently controllable so as to selectively control the gas flow into the turbine portions of the high-pressure turbocharger and the low-pressure turbocharger units. The valve members are asymmetric, e.g., they possess differing areas (e.g., perimeters, diameters and/or the like) with respect to one another.
However, a current challenge is to design a turbocharger system that provides active control, as opposed to passive control, of high boost pressure to the engine over a wide range of speed and load conditions. A still greater challenge to maintaining high boost and high engine torque through transient operation is the onset of series compounding from the low pressure stage turbocharger, especially with respect to the operation of the low pressure turbine portion.
Accordingly, there exists a need for new and improved regulated two-stage turbocharger systems that produce a relatively smooth and unnoticeable transition period between the operation of the high pressure and low pressure turbocharger units, especially with respect to the operation of the low pressure turbine portion.